This invention relates to bridge circuits for driving loads. Such bridge circuits find application in, for example, driving loads such as electric motors, which must be driven in opposite directions as desired.
In a typical known bridge circuit for driving an electric motor in an automotive application, the bridge circuit is formed by four field effect transistors connected in series pairs as the two parallel arms of the bridge. The voltage supply provided by the automobile battery is connected across the end-points of the two arms, and the mid-points of the two arms are connected across the electric motor. By switching the field effect transistors at two opposite sides of the bridge "on" and switching the other field effect transistors "off", current is driven through the motor in one direction; by switching the four field effect transistors to their opposite states, current is driven through the motor in an opposite direction.
However, such a known bridge circuit will be damaged if the battery voltage is connected to the bridge circuit with reverse polarity. The field effect transistors of the bridge are arranged so that the intrinsic diodes between their source and drain electrodes are reverse biased and no current flows when the battery voltage is connected to the bridge with the predetermined correct polarity. If the battery voltage is connected with the opposite polarity (as can easily occur in an automobile), the intrinsic diodes of the field effect transistors become forward biased and conduct: the transistors quickly burn out. Therefore, such known bridge circuits, if they are to be used in automotive applications, must be provided with additional reverse battery protection.